A pressure-sensitive recording material utilizing the reaction between a color former and a developer, for example, clay substances such as acidic clay, activated clay, attapulgite, zeolite, bentonite and kaolin, metal salts of aromatic carboxylic acids, and a phenol-formaldeyde resin is well known as described in, for example, U.S. Pat. Nos. 2,505,470, 2,505,489, 2,550,471, 2,548,366, 2,712,507, 2,730,456, 2,730,457, 3,418,250.
When, however, clay minerals such as acidic clay and activated clay are used as the developer, a serious problem arises in that a colored image is low in light fastness and moisture resistance.
A phenol-formaldehyde resin when used as the developer provides a colored image which is superior in light fastness and moisture resistance to that of the clay minerals. The phenol-formaldehyde resin, however, has the disadvantage that the colored image readily becomes yellow in color when exposed to light and nitrogen oxides.
On the other hand, the metal salts of aromatic carboxylic acids, when used as the developer, provide a colored image which is as high in light fastness and moisture resistance as that of the phenol-formaldehyde resin.
From the viewpoint of light fastness and moisture resistance of a colored image, therefore, it is desirable that the aromatic carboxylic acid methal salts be used as the developer. These aromatic carboxylic acid metal salts, however, have the disadvantage that fastness against platicizers is inferior to that of the clay minerals. Furthermore the light fastness of the colored image is not always sufficiently high.
In recent years, various pressure-sensitive recording sheets utilizing the combinations of indolyl-azaphthalide-based color former and clay minerals or phenol resin developers have been proposed in, for example, U.S. Pat. Nos. 4,275,905, 4,299,411, 4,046,776 and 4,564,679, British Patent Publication Nos. 2,039,934B, 2,082,194B, 2,040,303B, 1,467,098A, 2,075,042B, 2,103,234B and 2,148,923B, European Patent Publication Nos. 82822B and 140833A, Japanese Patent Publication Nos. 38243/76, Japanese Patent Application (OPI) Nos. 224582/85, and 139760/85 (the term "OPI" used herein means a "published unexamined patent application.").
It has been found that where the above-described aromatic carboxylic acid metal salts are used as the developer, if the above-described indolylazaphthalide compound is used as the color former, plasticizer resistance is improved as well as the light fastness and moisture resistance of the colored image. In this case, however, the hue and color density of a colored image are not sufficiently high.